Biomedical materials

ABSTRACT

A synthetic calcium phosphate-based biomedical material comprising gadolinium. The material may comprises a compound having the general chemical formula: Ca 10−y Gd y (PO 4 ) 6−x (SiO 4 )x(OH) 2−x+y  where 0&lt;x&lt;1.3 and 0&lt;y&lt;1.3.

The present invention relates to biomedical materials and, inparticular, to calcium phosphate bioceramics (e.g. apatite andhydroxyapatite) containing gadolinium as an MRI contrast agent.

The combined affects of an ageing population and greater expectations inthe quality of life have resulted in an increasing global demand fororthopedic implants for the replacement or augmentation of damaged bonesand joints. In bone grafting current gold standards include the use ofautograft and allograft but these methods are increasing. recognised asnon-ideal due to limitations in supply and consistency. Ceramics havebeen considered for use as bone graft substitutes to replace or extendtraditional bene grafts for over 30 years. In particular, calciumphosphates such as hydroxyapatite have been Promoted as a result oftheir osteoinductive properties

Accordingly, as surgical technique and medical knowledge continue toadvance, there has been a growth in the demand for synthetic bonereplacement materials. Consequently, there is an increasing interest inthe development of synthetic bone replacement materials for the fillingof both load bearing and non-load bearing osseous defects, such as injoint and facial reconstruction.

The biocompatibility of hydroxyapatite, coupled with the similaritiesbetween the crystal structure of hydroxyapatite and the mineral contentof bone, has led to great interest in hydroxyapatite as a material forthe augmentation of osseous defects. The apatite group of minerals isbased on calcium phosphate, with stoichiometric hydroxyapatite having amolar ratio of Ca/P of 1.67. Hydroxyapatite has the chemical formulaCa₁₀(PO₄)₆(OH)₂.

Silicate-substituted hydroxyapatite compositions provide attractivealternatives to stoichiometric hydroxyapatite as a bone replacementmaterial. Silicon has been shown to enhance the rate and quality of bonehealing when combined with calcium phosphate bone grafts, resulting infaster bone bonding between implant and host. PCT/GB97/02325 describes asilicate-substituted hydroxyapatite material.

Nuclear magnetic resonance (NMR) is the absorption of electromagneticradiation at a suitable precise frequency by a nucleus with anon-magnetic moment in an external magnetic field. NMR can be used forthe accurate determination of nuclear moments. It can also be used in asensitive form of magnetometer to measure magnetic fields. In medicine,magnetic resonance imaging (MRI) has been developed in which images oftissue are produced by magnetic-resonance techniques.

To enable clinicians to visualise tissue repair within a macroporouscalcium phosphate implant, or an implant with a cavity or void, withMRI, it is routine to introduce a gadolinium contrast agent by theinjection of a gadolinium-containing compound that will pass throughoutthe patient's blood stream. This can be associated with undesirable sideeffects and the process is also somewhat complicated and time-consuming.In addition, the process is not localised

The present invention aims to address least some of the problemsassociated with the prior art.

Accordingly, the present invention provides a synthetic calciumphosphate-based biomedical material comprising gadolinium.

At least some of the gadolinium is preferably in the form of Gd³⁺ ions.The gadolinium ion is believed to substitute for the calcium ion in thecalcium phosphate (e.g. apatite or hydroxyapatite) lattice.

Advantageously, the calcium phosphate composition according to thepresent invention contains gadolinium ions (e.g. Gd³⁺) that areincorporated into the calcium phosphate crystal structure. Thegadolinium ions in the calcium phosphate material enable it to act as acontrast agent in MRI.

The biomedical material preferably also further comprises silicon and/orsilicate. This has been found to improve the osteoinductivecharacteristics of the material. If present, the silicate ion isbelieved to substitute for the phosphate ion in the hydroxyapatitelattice.

For the avoidance of doubt, the term silicate-substituted as used hereinalso encompasses silicon-substituted. Likewise, silicon-substituted asused herein also encompasses silicate-substituted.

The calcium phosphate-based biomedical material will typically comprisehydroxyapatite or apatite.

In a preferred embodiment of the present invention, the materialcomprises a compound having the general chemical formula:Ca_(10−y)Gd_(y)(PO₄)_(6−x)(SiO₄)_(x)(OH)_(2−x+y)

Preferably, 0<x<1.3, more preferably 0.5<x<1.1.

Preferably, 0<y<1.3. It is also preferable that x≥y.

In one aspect, 0.001<y<1.1, more preferably 0.1<y<1.3. Such acomposition may be used diluted with another bone material (e.g. acalcium phosphate material), either synthetic or allograft or autograft,as an MRI contrast agent with gadolinium ions substituted into thecrystal structure of the biomedical material acting as the activecontrast agent.

In another aspect, 0<y<0.05, more preferably 0<y<0.025. Such acomposition may be Used undiluted.

In one embodiment of the present invention, where an essentially phasepure material is required, the phase purity of the material ispreferably at least 95%, more preferably at least 97%, still morepreferably at least 99%. In this case the material is substantially freeof any secondary phases. It will be appreciated that unavoidableimpurities may, however, be present. As will be appreciated, the phasepurity of the biomedical material can be measured by conventional X-raydiffraction techniques.

In another embodiment, where the presence of secondary phases does notpresent a problem, the material may further comprise one or moresecondary phases such as, for example, tricalcium phosphate (e.g. α-TCPand/or β-TCP), calcium silicate and tetracalcium phosphate. Accordingly,the present invention also provides for biphasic and multiphasematerials. The secondary phases may be present in an amount of up to 60wt. %, more typically up to 40 wt. %, still more typically up to 20 wt.%.

The present invention. also provides a biomedical material comprising asynthetic gadolinium-silicate co-substituted calcium phosphate-basedmaterial. Examples include gadolinium-silicate co-substitutedhydroxyapatite and apatite materials. The preferred features describedabove are also applicable either singularly or in combination to thisaspect of the present invention.

The biomedical material according to the present invention may be usedas a synthetic bone material, a bone implant, a bone graft, a bonesubstitute, a bone scaffold, a filler, a coating or a cement. Thebiomedical material may be provided in a porous or non-porous form. Thebiomedical material may be provided in the form of a composite material,for example in conjunction with a biocompatible polymer.

The present invention involves the synthesis of a calcium phosphatecomposition that contains gadolinium ions that are incorporated into thecalcium phosphate crystal structure. The amount of gadolinium ions thatare required in the calcium phosphate material to enable it to act as acontrast agent in MRI is quite low. The material according to thepresent invention can therefore be used in a number of ways as describedabove, i.e. undiluted or mixed with another bone material.

The biomedical material according to the present invention may beprepared by an aqueous precipitation method or a solid-state method suchas, for example, a hydrothermal method or a sol-gel method. The aqueousprecipitation technique is, however, preferred Accordingly, the presentinvention also provides a process for the synthesis of calciumphosphate-based material comprising gadolinium and optionally silicon,the process comprising: providing calcium or a calcium-containingcompound, a gadolinium-containing compound, a phosphorus-containingcompound and optionally a,silicon-containing compound; and forming aprecipitate by reacting the compounds in an aqueous phase at an alkalipH.

The process according to the present invention is preferably an aqueousprecipitation process.

The process may be used to synthesise hydroxyapatite and apatitematerials containing gadolinium and also preferably silicon.

Preferably, the calcium-containing compound comprises a calcium salt.The calcium salt may, for example, be selected from one or more ofcalcium hydroxide, calcium oxide, calcium chloride, calcium nitrateand/or calcium nitrate hydrate.

Preferably, the gadolinium-containing compound comprises one or both ofgadolinium chloride and/or gadolinium nitrate, preferably Gd(NO₃)₃.XH₂O.

The gadolinium is preferably present in the biomedical material (i.e.the final product) in an amount of up to 13 weight percent, morepreferably up to 12 weight percent. This is typical if the material isan essentially phase pure material as discussed above. On the otherhand, if the material comprises one or more secondary phases, then theamount of gadolinium in the material may exceed 13 weight percent andmay be present in an amount of up to 20 weight percent.

Preferably, the phosphorus-containing compound is selected from one orboth of a phosphate salt and/or a phosphoric acid. More preferably, thephosphorus-containing compound is selected from one or both of ammoniumphosphate and/or phosphoric acid.

Preferably, the (optional) silicon-containing compound comprises asilicate. More preferably, the silicate is selected from one or both oftetraethyl orthosilicate (TEOS) and/or silicon acetate.

The silicate is preferably present in the material (i.e. the finalproduct) in an amount Of up to 13 weight percent (which correlates to 4weight percent as silicon), more preferably up to 12 weight percent(which correlates to 3.66 weight percent silicon). Again, this istypical if the material is an essentially phase pure material asdiscussed above. On the other hand, if the material comprises one ormore secondary phases, then the amount of silicate in the material mayexceed 13 weight percent and may be present in an amount of up to 20weight percent.

In one embodiment, the silicon-containing compound and thegadolinium-containing compound are preferably supplied in substantiallyequimolar quantities with respect to the amount of silicon and thequantity of the gadolinium. In an alternative embodiment, thesilicon-containing compound is supplied in a greater molar quantity thanthe gadolinium-containing compound, with respect to the quantity ofsilicon and the quantity of the gadolinium.

The process according to the present invention is preferably carried outat an alkaline pH. Preferably, the pH is from 8 to 13. More preferably,the pH is from 10 to 12.

In order to adjust the pH of the solution to the desired pH, an alkaliis preferably added to ‘the solution. The alkali may be, for example,ammonium hydroxide or concentrated ammonia.

After the precipitate has been formed it may be dried, heated and/orsintered. The precipitate may be heated and/or sintered to a temperaturein the range of from 800° C. to 1500° C., preferably from 1000° C. to1350° C., more preferably from 1200° C. to 1300° C.

The process according to the present invention preferably comprisesfirst forming under ambient conditions (although this step can beperformed at a temperature up to about 100° C.) an aqueous suspensioncomprising calcium or the calcium-containing compound (e.g. Ca(OH)₂) andthe gadolinium-containing compound (e.g. Gd(NO₃)₃.XH₂O). Next, anaqueous solution of the phosphorus-containing compound (e.g. H₃PO₄) isslowly added to the suspension with stirring. Finally, and if desired,an aqueous solution of the silicon-containing compound (e.g. Si(OC₂H₅)₄TEOS) is added slowly with stirring (instead, the aqueous solution ofthe phosphorus-containing compound can be mixed with the aqueoussolution of the silicon-containing compound, and the combination thenadded slowly with stirring the suspension). The pH is monitored andmaintained at an alkali pH, preferably from 11 to 13, using, forexample, concentrated ammonia solution. The total mixture is then leftto age and precipitate, which typically takes up to 12 to 24 hours.

Once the precipitate has been formed and aged it may be dried, heatedand/or sintered. Preferably, it is first dried by heating it to atemperature of up to 100° C. This may then be followed by heating atemperature in the range of from 800° C. to 1500° C., more preferablyfrom 1000° C. to 1350° C., and even more preferably from 1200° C. to1300° C., in order to sinter the material. The dried precipitate ispreferably ground into a powder prior to the sintering step.

The aforementioned process may be used to prepare an essentially phasepure material as herein described.

If it is desired to produce a biphasic or multiphase material, a numberof steps may be used independently or in combination. For example, adeficient amount of the calcium-containing compound (e.g. Ca(OH)₂) andthe gadolinium-containing compound (e.g. Gd(NO₃)₃.XH₂O) may be added,which promote the precipitation of a gadolinium/silicate co-substitutedcation-deficient apatite composition. This will form a biphasicgadolinium/silicate co-substituted composition on heating.

The process according to the present invention may involve diluting thethus formed calcium phosphate-based gadolinium-containing material withanother material (e.g. a gadolinium-free material) to produce asufficient gadolinium cation concentration that will act as a contrastagent for MRI. The gadolinium-free material may be, for example, acalcium phosphate-based material or polymer material. Alternatively, itmay be allograft or autograft bone, demineralised bone matrix, or asynthetic or natural material for the repair or replacement of bone.

The present invention also provides a method of improving. MRI contrastin a calcium phosphate material (e.g. hydroxyapatite or apatite), whichmethod comprises substituting Gd³⁺ into the lattice.

The present invention also provides a method of improving MRI contrastin a silicon-substituted calcium phosphate material (e.g. hydroxyapatiteor apatite), which method comprises co-substituting Gd³⁺ into thelattice.

The present invention also provides for the use of gadoliniumsubstituted in a synthetic hydroxyapatite or apatite lattice to improveMRI contrast.

The present invention also provides for the use of gadoliniumco-substituted with silicate in a synthetic hydroxyapatite or apatitelattice to improve MRI contrast.

The present invention will now be described further with reference tothe following preferred embodiments.

To enable an improvement in MRI contrast the present invention providesa substitution method, whereby gadolinium is substituted (orco-substituted with silicate ions) into the calcium phosphate lattice(e.g. hydroxyapatite or apatite).

Two distinct Mechanisms are possible. The first essentially negates theloss of hydroxyl (OH) groups from the hydroxyapatite lattice to balancecharge, as the equimolar substitution of gadolinium and silicate ionsfor calcium and phosphate ions balances charge:Gd³⁺+SiO₄ ⁴⁻→Ca²⁺+PO₄ ³⁻  (1)

This may be summarised by the general equation:Ca_(10−x)Gd_(x)(PO₄)_(6−x)(SiO₄)_(x)(OH)₂

The second substitution, where proportionally less gadolinium ions areco-substituted with silicate ions, which results in the reduction in thelevel of OH groups to balance the charge (for an example where the molaramount of gadolinium is half the molar amount of silicate beingsubstituted):0.5Gd³⁺SiO₄ ⁴⁻→0.5Ca²⁺+PO₄ ³⁻+0.5OH⁻  (2)

This may be summarised by the general equation:Ca_(10−y)Gd_(y)(PO₄)_(6−x)(SiO₄)_(x)(OH)_(2−x+y)   (2)(for this mechanism, y<x)

As for other ionic substitutions in the hydroxyapatite lattice, thesesubstitutions have limits. When the levels of substitution pass theselimits, the composition becomes thermally unstable during sintering, andphase decomposition of the hydroxyapatite occurs, leading to theformation of secondary phases. The co-substituted method according tothe present invention may be used to increase the limit of silicatesubstitution into the hydroxyapatite lattice, without resulting in phasedecomposition after sintering at typical temperatures (approximately1200° C. or more), to a maximum of approximately 12 wt. % silicate ions(or 3.66 wt. % silicon). After this limit has been reached, secondaryphases are produced. For certain applications, an essentially phase purematerial is desired, and so the maximum of approximately 12 wt. %silicate ions will be adopted. For other applications, the presence ofsecondary phases may actually be desired may not present problem), andthe limit of approximately 12 wt. % silicate may then be exceeded.

The present invention will now be further described with reference tothe following Examples and the accompanying drawings, provided by way ofexample, in which:

FIG. 1 is an X-ray diffraction pattern obtained for Example 1 comparedwith the ICDD (#09-0432) standard pattern for hydroxyapatite;

FIG. 2 is an X-ray diffraction pattern obtained for Example 2 comparedwith the ICDD (#09-0432) standard pattern for hydroxyapatite; and

FIG. 3 is an X-ray diffraction pattern obtained for Example 3 comparedwith the ICDD (#09-0432) standard pattern for hydroxyapatite and(#09-0359) standard pattern for alpha-tricalcium phosphate.

EXAMPLE 1 Synthesis of Gd³⁺/SiO₄ ⁴⁻ Co-Substituted hydroxyapatite(x=y=1.0)

The following method describes the synthesis of approx. 10 g of aGd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite by an aqueous precipitationmethod with the following substitution mechanism:Ca_(10−y)Gd_(y)(PO₄)_(6−x)(SiO₄)_(x)(OH)_(2−x+y)where x=y =1.0

CaCO₃ was first de-carbonated overnight (16 hours) in a furnace at 900°C. The resulting CaO was then removed from the furnace and placed in adessicator to cool. 5.0543 g CaO was added to a beaker containingapprox. 100 ml deionised water in an ice bath. After complete additionof the CaO, the beaker was removed from the ice bath and placed on astirrer. The suspension was left to stir for approx. 10 minutes; the CaOwill undergo hydration to from Ca(OH)₂.

Meanwhile, 4.5141 g Gd(NO₃).6H₂O (GNH) was added to a beaker containingapprox. 100 ml deionised water and mixed well until the GNH hadcompletely dissolved. The GNH solution was then slowly poured into theCa (OH) ₂ suspension and this suspension was left stir for approx. 15minutes.

5.7664 g H₃PO₄ (85% assay) was diluted with approx. 100 ml deionisedwater. This solution was poured into a dropping funnel and addeddrop-wise to the stirring Ca(OH)₂/GNH suspension over a period ofapprox. 70 minutes. After complete addition of the H₃PO₄ solution, 2.127g Si(OC₂H₅)₄ (TEOS) was diluted with approx. 100 ml deionised water andthis solution was poured into a dropping funnel and added drop-wise tostirring Ca(OH)₂/GNH/H₃PO₄ mixture over a period of approx. 70 minutes.The pH of the stirring solution was monitored throughout the addition ofthe H₃PO₄ and TEOS solutions and was maintained at pH12 by the additionof concentrated ammonia solution; in total, approx. 100 ml was added.After complete addition of the TEOS solution, the total mixture was leftto stir for a further 2 hours before being left to age and precipitateovernight (approximately 16 hours). The Precipitate was then filtered,dried at 80° C. for 24 hours, and ground to form a fine powder.Approximately 3 g of the dried powder was placed in a platinum crucibleand sintered in a furnace at 1200° C. for 2 hours, using heating andcooling rates of 5 and 10° C./min respectively. The sintered powder wasthen analysed using X-ray diffraction to confirm the phase purity. ABruker D8 diffractometer was used to collect data from 25 to 40° 2 θwith a step size of 0.02° and a count time of 9.5 secs/step. Thediffraction pattern obtained was compared with the ICDD (#09-0432)standard pattern for hydroxyapatite. All the diffraction peaks for thesintered Gd ³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite matched the peaksof the ICDD standard, with no additional peaks observed, indicating thatthe composition produced by this method was a single-phase material witha hydro structure (see FIG. 1).

0.2503 g of this Gd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite was dilutedby mixing, in a pestle and mortar, with 4.7505 g of a SiO₄ ⁴⁻substituted hydroxyapatite to give a powder containing 5 wt % Gd³⁺/SiO₄⁴⁻ co-substituted hydroxyapatite. Similarly, 0.0124 g of the Gd³⁺/SiO₄⁴⁻ co-substituted hydroxyapatite was mixed with 4.9879 g of a SiO₄ ⁴⁻substituted hydroxyapatite to give a powder containing 0.25 wt %Gd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite. Approx. 5 ml deionised waterwas added to each of these powders to form pastes. Magnetic ResonanceImaging (MRI) was then used to assess the effect of Gd³⁺ substitution onthe MRI activity of the hydroxyapatite materials. Materials that havelittle or no MRI activity appear as darkened areas in the resultingimage. Those that are MRI active will in contrast appear bright. Thesample containing 5 wt % Gd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite wasfound to have an improved contrast, i.e. appeared brighter, than a purehydroxyapatite. However, the contrast of the 0.25 wt % Gd³⁺/SiO₄ ⁴⁻co-substituted hydroxyapatite was found to be even greater than that ofthe 5 wt % Gd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite.

EXAMPLE 2 Synthesis of Gd³⁺/SiO₄ ⁴⁻ Co-Substituted hydroxyapatite(x=1.0, y=0.5)

The following method describes the synthesis of approx. 10 g of aGd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite by an aqueous precipitationmethod with the following substitution mechanism:Ca_(10−y)Gd_(y)(PO₄)_(6−x)(SiO₄)_(x)(OH)_(2−x+y)where x=1.0, y=0.5

CaCO₃ was first de-carbonated overnight (16 hours) in a furnace at 900°C. The resulting CaO was then removed from the furnace and placed in adessicator to cool. 5.3284 g CaO was-added to a beaker containingapprox. 100 ml deionised water in an ice bath. After complete additionof the CaO, the beaker was removed from the ice bath and placed on astirrer. The suspension was left to stir for approx. 10 minutes; the CaOwill undergo hydration to from Ca(OH)₂. Meanwhile, 2.2589 g Gd(NO₃).6H₂O(GNH) was added to a beaker containing approx. 100 ml deionised waterand mixed well until the GNH had completely dissolved. The GNH solutionwas then slowly poured into the Ca(OH)₂ suspension and this suspensionwas left to stir for approx. 15 minutes.

5.7645 g H₃PO₄ (85% assay) was diluted with approx. 100 ml deionisedwater. This solution was poured into a dropping funnel and addeddrop-wise to the stirring Ca(OH)₂/GNH suspension over a period ofapprox. 90 minutes. After complete addition of the H₃PO₄ solution,2.1330 g Si(OC₂H₅)₄ (TEOS) was diluted with approx. 100 ml deionisedwater and this solution was poured into a dropping funnel and addeddrop-wise to stirring Ca(OH)₂/GNH/H₃PO₄ mixture over a period of approx.75 minutes. The pH of the stirring solution was monitored throughout theaddition of the H₃PO₄ and TEOS solutions and was maintained at pH12 bythe addition of concentrated ammonia solution; in total, approx. 50 mlwas added. After complete addition of the TEOS solution, the totalmixture was left to stir for a further 2 hours before being left to ageand precipitate overnight (approximately 16 hours). The precipitate wasthen filtered, dried at 80° C. for 24 hours, and ground to form a finepowder.

Approximately 3 g of the dried powder was placed in a platinum crucibleand sintered in a furnace at 1200° C. for 2 hours, using heating andcooling rates of 5 and 10° C./min respectively. The sintered powder wasthen analysed using X-ray diffraction to confirm the phase purity. ABruker D8 diffractometer was used to collect data from 25 to 40° 2 θwith a step size of 0.02° and a count time of 9.5 secs/step. Thediffraction pattern obtained was compared with the ICDD (#09-0432)standard pattern for hydroxyapatite. All the diffraction peaks for thesintered Gd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite matched the peaks ofthe ICDD standard, with no additional peaks observed, indicating thatthe composition produced by this method was a single-phase material witha hydroxyapatite-like structure (see FIG. 2).

EXAMPLE 3 Synthesis of Gd³⁺/SiO₄ ⁴⁻ Co-Substitutedhydroxyapatite/tricalcium Phosphate Biphasic Mixture (x=1.1, y=1.1)

The following method describes the synthesis of approx. 10 g of aGd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite/tricalcium phosphate biphasicmixture by an aqueous precipitation method with the followingsubstitution mechanism:Ca_(10−y)Gd_(y)(PO₄)_(6−x)(SiO₄)_(x)(OH)_(2−x+y)where x=1.1, y=1.1

To produce a gadolinium/silicate co-substituted biphasic composition,rather than a single phase gadolinium/silicate co-substitutedhydroxyapatite composition (described in Example 1), a deficient amountof Ca(OH)₂ and GNH was added, which promoted the precipitation of agadolinium/silicate co-substituted cation-deficient apatite composition.This will form a phasic gadolinium/silicate co-substituted compositionon heating.

CaCO₃ was first de-carbonated overnight (16 hours) in a furnace at 900°C. The resulting CaO was then removed from the furnace and placed in adessicator to cool. 4.9978 g CaO, was added to a beaker containingapprox. 100 ml deionised water in an ice bath. After complete additionof the CaO, the beaker was removed from the ice bath and placed on astirrer. The suspension was left to stir for approx. 10 minutes; the CaOwill undergo hydration to form Ca(OH)₂. Meanwhile, 4.9696 g Gd(NO₃)₃.6H₂0 (GNH) was added to a beaker containing approx. 100 ml deionised waterand was mixed until the GNH had completely dissolved. The GNH solutionwas then slowly poured into the Ca (OH) ₂ suspension and this suspensionwas left to stir for approx. 15 minutes.

5.6500 g H₃PO₄ (85% assay) was diluted with approx. 100 ml deionisedwater and this solution was poured into a dropping funnel and addeddrop-wise to the stirring Ca(OH)₂/GNH suspension over a period ofapprox. 80 minutes. After complete addition of the H₃PO₄ solution,2.3402 g Si(OC₂H₅)₄ (TEOS) was diluted with approx. 100 ml deionisedwater and this solution was poured into a dropping funnel and addeddrop-wise to stirring Ca(OH)₂/GNH/H₃PO₄ mixture over a period of approx.75 minutes. The pH of the stirring solution was monitored throughout theaddition of the H₃PO₄ and TEOS solutions and was maintained at pH12 bythe addition of concentrated ammonia solution; in total approx. 50 mlwas added. After complete addition of the H₃PO₄ and TEOS solutions, thetotal mixture was left to stir for a further 2 hours before being leftto age and precipitate overnight (approximately 16 hours). Theprecipitate was then filtered, dried at 80° C. for 24 hours, and groundto form a fine powder. Approximately 3 g of the dried powder was placedin a platinum crucible and sintered in a furnace at 1200° C. for 2hours, using heating and cooling rates of 5 and 10° C./min,respectively.

The sintered powder was then analysed using X-ray diffraction to confirmthe phase purity. A Broker D8 diffractometer was used to collect datafrom 25 to 40° 2 θ with a step size of 0.02° and a count time of 9.5secs/step. The diffraction pattern obtained was compared with the ICDD(#09-0432) standard pattern for hydroxyapatite and (#09-0359) standardpattern for alpha-tricalcium phosphate. All the diffraction peaks forthe sintered Gd³⁺/SiO₄ ⁴⁻ co-substituted hydroxyapatite matched thepeaks of the ICDD standard, matching both the hydroxyapatite phase andthe alpha-tricalcium phosphate phase, indicating that the compositionproduced by this method was a biphasic materials (see FIG. 3). Comparingthe intensities of the most intense peaks of the hydroxyapatite and thealpha-tricalcium phosphate phases, the amount of hydroxyapatite isapproximated as 84%, and the amount of tricalcium phosphate as 16%.

EXAMPLE 4 Use of a Gd³⁺/SiO₄ ⁴⁻ Co-Substituted hydroxyapatite (x=y=1.0)in a Calcium Phosphate Composition to Enhance MRI Contrast in a BoneGraft Material

To enable clinicians to visualise tissue repair within a macroporouscalcium phosphate implant, or an implant with a cavity or void, withMRI, a surgeon would have to introduce a gadolinium contrast agent,normally by the injection of a gadolinium-containing compound that willpass throughout the patient's blood stream. The present inventionprovides for the synthesis of a calcium phosphate composition thatcontains gadolinium ions that are actually incorporated into the calciumphosphate crystal structure. The amount of gadolinium ions that arerequired in the calcium phosphate material to enable it to act as acontrast agent in MRI is quite low. The materials and methods describedherein can therefore be used in one of two ways. Firstly, a calciumphosphate material such as that described in Example 1 can be preparedbut with a very small value of x, typically x=0.0001 to. 0.05; thismaterial could then be used to construct the entire implant, and wouldcontain sufficient gadolinium to act as a contrast agent in MRI.Alternatively, a calcium phosphate material such as that described inExample 1 can be prepared with a large value of x, such as x=1. Thismaterial could then be mixed with an appropriate amount ofgadolinium-free calcium phosphate implant material, to effectivelydilute the gadolinium-substituted calcium phosphate (with x=1) to anappropriate final concentration of gadolinium, for example in a 1:1000ratio by weight of gadolinium-substituted calcium phosphate (with x=1)to gadolinium-free calcium phosphate implant material. Alternatively,the gadolinium-substituted calcium phosphate (e.g. with x=1) could bemixed with another synthetic bone replacement material, or withautograft or allograft bone, or with another bone replacement materialof natural or synthetic origins.

As an example, the following compositions were prepared:

Samples (5 g), as sintered powders with particle sizes ranging from 10to 500 microns, were placed in a small plastic sealable bag, and 7.5 gof water was added to form a paste like consistency. The bag was sealed,rolled up to form a cylindrical shaped sample, then covered in clingfilm. The samples were then fixed with tape onto a plastic bottlecontaining a dilute nickel chloride phantom solution, and placed in a 1Tesla MRI. Gadolinium-substituted calcium phosphate with x=1 (asdescribed in Example 1) was mixed with gadolinium-free calcium phosphateto provide a final concentration of Gd of approximately 0.25 wt % (A) Asa comparison, a Gadolinium-substituted calcium: phosphate. (as describedin Example 1) was prepared with x=0.02, corresponding to a gadoliniumcontent of approximately 0.25 wt % (B). As controls, samples wereprepared of gadolinium-free calcium phosphate (C) andGadolinium-substituted calcium phosphate with x=1 (as described inExample 1) mixed with gadolinium-free calcium phosphate to provide afinal concentration of Gd of approximately 5 wt % (D). The contrastobserved for samples A and B were similar and were significantlyenhanced compared to the two control samples C and D.

The present invention enables gadolinium to be substituted (or cosubstituted with silicon) into the calcium phosphate (e.g.hydroxyapatite or apatite) lattice. As a consequence, the MRI contrastof the material can be improved. The synthetic biomedical material stillclosely matches the chemical composition of bone mineral. The presentinvention also enables the production of phase-pure, biphasic andmultiphase materials.

What is claimed is:
 1. A process for the synthesis of a calciumphosphate-based material comprising gadolinium and silicon, the processcomprising: providing calcium or a calcium-containing compound, agadolinium-containing compound, a phosphorus-containing compound and asilicon-containing compound; and forming a precipitate by reacting thecompounds in an aqueous phase at an alkali pH; thereby synthesizing acalcium phosphate-based material comprising gadolinium, wherein at leastsome of the gadolinium is in the form of Gd³⁺ ions substituted in thecalcium phosphate lattice, and wherein the material comprises a compoundhaving the general chemical formula:Ca_(10−y)Gd_(y)(PO₄)_(6−x)(SiO₄)_(x)(OH)_(2−x+y) wherein 0.5<x<1.1 and0<y<1.3.
 2. The process of claim 1, wherein the calcium-containingcompound comprises a calcium salt.
 3. The process of claim 2, whereinthe calcium salt is selected from one or more of calcium hydroxide,calcium chloride, calcium nitrate and/or calcium nitrate hydrate.
 4. Theprocess of claim 1, wherein the gadolinium-containing compound isselected from one or both of gadolinium chloride and/or gadoliniumnitrate.
 5. The process of claim 1, wherein the phosphorus-containingcompound is selected from one or both of a phosphate salt and/or aphosphoric acid.
 6. The process of claim 5, wherein thephosphorus-containing compound is selected from one or both of ammoniumphosphate and/or phosphoric acid.
 7. The process of claim 1, wherein thesilicon-containing compound comprises a silicate.
 8. The process ofclaim 7, wherein the silicate is selected from one or both of tetraethylorthosilicate (TEOS) and/or silicon acetate.
 9. The process of claim 1,wherein the pH is from 8 to
 13. 10. The process of claim 1, wherein analkali is added to adjust the pH of the solution to the desired pH. 11.The process of claim 10, wherein the alkali is ammonium hydroxide orconcentrated ammonia.
 12. The process of claim 1, wherein after theprecipitate has been formed it is dried, heated and/or sintered.
 13. Theprocess of claim 1, wherein the silicon-containing and thegadolinium-containing compounds are supplied in equimolar quantitieswith respect to the amount of silicon and the quantity of the gadoliniumcation.
 14. The process of claim 1, wherein the silicon-containingcompound is supplied in a greater quantity than the gadolinium compound,with respect to the quantity of silicon and the quantity of thegadolinium cation.
 15. The process of claim 1, wherein the processfurther involves diluting the thus formed calcium phosphate-basedgadolinium-containing material with a gadolinium-free material.
 16. Theprocess of claim 1, wherein the phase purity of the material, asmeasured by X-ray diffraction using the whole pattern method, is atleast 95%.
 17. The process of claim 1, wherein 0.001<y <1.1.
 18. Theprocess of claim 1, wherein 0.1<y <1.3.
 19. The process of claim 1,wherein 0<y <0.001.
 20. The process of claim 1, wherein 0<y <0.0005. 21.The process of claim 1, wherein x≥y.